MARS Recumbent bicycle projects

Zotefoam Manual - Internal Braces
A project by John Tetz - April 2005

Introduction   Mold   Layout   Form   Cut   Glue   Internal Braces   Install the Trike   Tail   Door   Canopy   Turtledeck

The foamshell by itself is floppy – one can’t get this kind of lightness without some floppiness. By adding internal braces and attaching the foamshell to a trike (or bike) frame, the foamshell becomes surprisingly stiff. I used two types of internal braces: U-shaped and tongue & groove. All the braces for this foamshell were made from various thicknesses of HD 80 Zotefoam.

To form the U-shaped braces, I made wooden male and female molds. I used 0.2” thick HD 80 for the bigger braces and 0.175” HD 80 for the smaller braces, so order a sheet each of 0.2" and 0.175" HD 80 foam. The smaller braces get plugged into the bigger braces to align the removable tail with the main shell. Here is a drawing of the molds to make these braces:
 
The tongue & groove braces are used to support the edges at the door/canopy, tail/shell, and turtledeck/shell junctions. This shows a sectional view of a typical edge support:
This shows the layout of the braces used on my VeloTrike; use this drawing as a main reference.

To form the U-shaped braces, heat about 8” of Zotefoam and lay it on the female mold. Press the male mold down into the Zotefoam and push it into the female mold. This can be done by hand. Hold for 20 seconds, leaving the male form in the mold. Heat the next 8” of Zotefoam sticking out beyond the mold. Then lift the male form out and move the heated Zotefoam over the mold. Press the male back into the Zotefoam and hold for about 10 seconds. As this section is cooling, heat the next section. Repeat the process to make long pieces. After the entire length is pre-formed, do a second pass to make a cleaner, sharper U-shaped form.

With a razor blade, trim the sides of the U uniformly. The step in the mold allows the razor blade to lie flat. Then glue in a bottom piece of HD 80 to close the loop, using the usual gluing techniques. The bottom increases the stiffness, plus it gives a bigger gluing area for the inside of the shell. This assembly is extremely stiff yet very light.
 

The following work is done inside the hanging shell:

Attach the first internal brace along the left side at the 22” line at the bottom of the door. This brace replaces some of the stiffness in the shell that is lost when the door is cut out. I had suggested cutting two marks with a razor blade when the shell was on the mold. Stick two razors in these slots and you will see where the internal line for the brace has to go. Run the brace from the nose area to the tail cutoff. This brace is low enough that your shoes will not rub up against it. Pre-curve the brace to fit the shell shape (cold form it). Hold the brace up against the inside of the shell and draw glue lines on either side of it. Sand this area and use the double glue technique to glue it in place. Lay down masking tape on either side of the lines to prevent the glue from going beyond the edge of the stiffeners. This makes a cleaner looking installation.

 
A pair of braces should straddle the green/grey line along the bottom. This line runs from the glass nose, interrupted by the spray shields around the front wheel openings, and continues to the end of the main shell. This is will be the lower alignment coupling to the tail.

Another brace is needed on the right side at the 27.5” line. This cannot go all the way to the nose because it’s high and your toe would hit it. This brace stops at the rear end of the canopy. An alignment slot will go from the end of the canopy to the shell/tail junction (described later).

With one U-shaped brace at the green/grey bottom line on the right side, a second U-shaped brace is needed halfway from this bottom line to the 27.5” line. This could be a short one, about 15” long from the rear end of the shell toward the front.

Drawing #3 above shows the layout positions of the tongue & groove braces that will be used on the shell/tail, door/canopy, turtle deck/shell and tail junctions, which will be described later in sections Tail, Door, and Canopy.

Next: Installing the trike

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